I use an X-Fi Xtreme Music for gaming, which has CMSS 3D, which essentially is very similar to Dolby Headphone in what it does. I do also have a Xonar, which is in my media PC, but I have used it for gaming on the odd occasion, just to see what DH is like for gaming. Both CMSS 3D and DH work very well for positional sound in games, using stereo headphones, but DH I find is slightly superior due to my headphones (Goldring DR150's) being more suited to DH than CMSS 3D. One area where DH completely outshines CMSS 3D is when watching films if headphones are used.
DH and CMSS 3D uses algorithms and cues to fool the brain into thinking the sound is coming from a specific location, which is virtual. This seems to bug some people who are fans of 5.1/7.1 headphones, as they use multiple drivers in each ear cup, so the sound is really coming from the rear of the ear cup for eg. However, 5.1/7.1 is clever marketing really by the companies that sell them. A 7.1 speaker layout, uses side speakers, in headphones how does that differ from 5.1 though? as the drivers at placed at the side of ones head anyway, so it's not as if 5.1 headphones don't have side speakers/drivers. Some fans of 5.1/7.1 headphones call them 'true' and what DH provides etc, is merely virtual. A true 5.1/7.1 speaker system involves careful speaker placement, and the sound reacts with the room and with the users body. How is slapping multiple drivers onto either side of ones head 'true' 5.1/7.1?
5.1/7.1 headphones use multiple drivers, whereas stereo headphones use 2. Which will provide better sound quality? a bunch of cheaper drivers, or 2 better quality ones?
For anyone who doesn't give a flying monkeys about sound quality, and doesn't want to buy a gaming sound card, then 5.1/7.1 headphones should suit them fine. So they do have their place. For anyone with a Xonar DX/D1 or above, or a X-Fi Xtreme Gamer/Music or above, to go and buy 5.1/.7.1 headphones, makes that sound card a worthless purchase IMO, if they are only going to use headphones.
Goldring DR150 or NS1000 are worth considering, they are great headphones and have very good synergy with Dolby Headphone. Sennheiser's HD555/595 seem to have a better synergy with CMSS 3D found on X-Fi's, however they will work fine with DH, just as my Goldring's work fine CMSS 3D, despite having a better synergy with DH.
Just go for the ones that appeal to you, or you wallet the most. I will say though, that the HD595's at £120-130 are expensive IMO. When you consider the Goldring's can be bought for half that, makes me wonder if the HD595 are with £120-130. Yes, they are better, but worth double? I'm not so sure. Of course though, we all have different ears, and the way we perceive sound is different. Might explain why some do like multiple driver headphones, but from what I have seen, the majority prefer DH/CMSS 3D + stereo headphones when they have tried both solutions. Some might feel the £120-130 price tag of the HD595 is justified, which is fair enough if the person is happy with them and the price paid, as they are certainly very good headphones.
